1. Field of Invention
The present invention relates to a motor structured such that magnetization of the rotor thereof and layout of polar teeth of the stator thereof are devised to realize a high speed and large torque, and more particularly to a motor for use as a stepping motor.
2. Relate Art
Stepping motors are widely used in a variety of industrial fields. The stepping motor must realize high speed and large torque. To realize the high speed and large torque, efficient exertion of the magnetic flux generated in the polar teeth on the magnet in the rotor so as to raise the efficiency of use of the magnet is an important factor.
FIG. 14 shows the structure of a conventional stepping motor which is schematically constituted by a rotor 2 having a rotational shaft 1; and a stator portion 3 disposed to surround the rotor 2. The rotor 2 is constituted by a cylindrical boss 4 and a cylindrical magnet 5 disposed around the boss 4.
The stator portion 3 incorporates cores 6a and 6b disposed to form two stages and arranged t serve as stator members; coils 7a and 7b wound around the cores 6a and 6b and arranged to serve as wound coils; and stator caps 8a and 8b which are stator members also serving as caps. The opposite surfaces of the cores 6a and 6b and the magnet 5 of the stator caps 8a and 8b are provided with polar teeth T11, T12, . . . , T21, T22, . . . , (refer to FIG. 15 or 16).
The rotor 2 and the stator portion 3 are held by a joining plate 9 and an upper cover 10 joined to the two ends (in the vertical direction in FIG. 14) of the rotor 2. The joining plate 9 and the upper cover 10 are provide with bearings 11a and 11b. The foregoing rotational shaft 1 is rotatively supported by the bearings 11a and 11b. A disc spring 12 is sandwiched between the rotor 2 and the bearing 11a, while a washer 13 is sandwiched between the rotor 2 and the bearing 11b.
FIG. 15 is a cross section l view taken along line indicated with arrows X--X shown in FIG. 1 and arranged to show the layout of polar teeth provided for the cores 6a and 6b of the stator portion 3 and the stator caps 8a and 8b. As can be understood from FIG. 15, the polar teeth T11 and T13 of the polar teeth T11, T12, T13 and T14 are provided for the core 6a of the stator portion 3. The polar teeth T12 and T14 are provided for the stator cap 8a. That is, the polar teeth T11, T12, T13 and T14 are alternately provided for the core 6a and the stator cap 8a. Also the core 6b and the stator cap 8b are similarly structured.
FIG. 16 is a cross sectional view taken along a line indicated with arrows Y--Y shown in FIG. 14. The relationship among the polar teeth of the core 6b and the stator cap 8b and the magnetized segments of the magnet 5 is schematically shown.
As shown in FIG. 16, the palar teeth T21, T22, T23, T24, . . . , are disposed apart from the outer surface of the magnet 5 for a predetermined distance to form a line such that the polar teeth are mutually engaged to one another in the circumferential direction of the magnet 5. As described above, the polar teeth T22 and T24 are provided for the core (the core 6b in the foregoing case) and the polar teeth 21 a d T23 are provided for the stator cap (the stator cap 8b in the foregoing case). That is, the polar teeth are alternately provided for the core 6b and the stator cap 8b.
The foregoing polar teeth T21, T22, T23, T24, . . . , are magnetized in such a way that the adjacent polar teeth are magnetized to opposite polarities. For example, the polar tooth T21 is magnetized to the south pole, the polar tooth T22 is magnetized to the north pole, the polar tooth T23 is magnetized to the south pole and the polar tooth T24 is magnetized to the north pole. Thus, the magnet 5 is brought to a state in which attraction and repulsion forces are exerted on the magnetized segments of the surfaces opposite to the polar teeth T21, T22, T23, T24, . . . , so that the rotor 2 is rotated.
The conventional stepping motor having the above-mentioned structure, however, encounters a problem in that the efficiency of using the magnetic flux gene rated in the stator portion 3 is unsatisfactory. That is, as shown in FIG. 17, flows of the magnetic flux generated by the polar teeth T21, T22, T23, T24, . . . , include magnetic flux Hi which is exerted on the magnet 5 and a multiplicity of leaked magnetic flux portions, such as magnetic flux H2 which flows along the reverse surface of the polar teeth T21, T22, T23, . . . , and magnetic flux H3 which flows in between adjacent polar teeth.
Therefore, only a portion of the magnetic flux which is exerted on the magnet 5 is used. That is, the effective magnetic flux which is exerted on the magnet 5 to rotate the rotor 2 is only the magnetic flux H1 in a case of the structure shown in FIG. 17. The other flux is wasted. Therefore, only the magnetic flux Hi which is exerted on the mag et 5 is used as the output of the motor. As a result, a satisfactory efficiency cannot be realized.
The conventional structure incorporates the polar teeth T11, T12, . . . , T21, T22, . . . , arranged in a line such that the adjacent polar teeth have different polarities. Therefore, gap Gi (see FIG. 15) must be formed between polar teeth. Therefore, design of the polar teeth is considerably limited. The foregoing fact will now be described with reference to FIG. 15. Each of the polar teeth T11, T12, . . . , T21, T22, . . . , has the trapezoidal shape. Both of the width W1 of the leading end of the trapezoid and the width W2 of the base of the same must be not larger than magnetizing pitch of the magnet 5. Moreover, gap G1 between the polar teeth having a size similar to the thickness (L1) of each of the stator caps 8a and 8b must be provided. To meet the foregoing requirements, the size, shape and the arrangement of the polar teeth are inevitably limited. Therefore, design of the stepping motor is considerably limited.